(i) Field of the Invention
The present invention relates to a process for partial oxidation, in a Claus furnace, of a gas stream containing hydrogen sulphide, by reaction with a stream of a gas such as oxygen-enriched air.
(ii) Description of Related Art
Hydrogen sulphide-rich gas streams are wastes produced by many industries, especially the oil refining industry and the production of natural gas. For reasons connected especially with the environment, these hydrogen sulphide-rich gases cannot be released as such into the atmosphere. It is therefore necessary to treat them with a view to appreciably reducing their hydrogen sulphide content. A well-known process for treating these hydrogen sulphide-rich gases is the Claus process.
This process includes a thermal part and a catalytic part. In the thermal part two main reactions are carried out. The first reaction consists in reacting approximately a third of the hydrogen sulphide to be treated with oxygen, to produce water and sulphur dioxide according to the following reaction: EQU H.sub.2 S+3/2O.sub.2 .fwdarw.H.sub.2 O+SO.sub.2.
The remaining 2/3 of the hydrogen sulphide to be treated are reacted with the sulphur dioxide formed during the above first stage, according to the following reaction: EQU SO.sub.2 +2H.sub.2 S.fwdarw.2H.sub.2 O+3S.
The first reaction is usually carried out with atmospheric oxygen. In order to increase the production efficiency of the thermal part of the Claus process it has already been proposed to react, during the said first stage, hydrogen sulphide with oxygen-enriched air. However, it is generally considered that this enrichment of air with oxygen must not exceed a certain threshold because otherwise the heat of the reaction is such that the temperature at the walls of the Claus furnace exceeds the temperature which can be withstood by the refractories forming part of these walls. In this respect reference may be made to Patent Application EP-A-165,609, which describes that, if a gas stream containing 90 mol % of hydrogen sulphide is reacted with a gas stream containing 40 mol % of oxygen, the theoretical flame temperature, calculated in the adiabatic conditions, should reach approximately 1732.degree. C. In the same conditions, but with a gas stream containing 70 mol % of oxygen, it is indicated that the flame temperature should reach approximately 2065.degree. C. The refractories which give the highest performance are described in this document as not capable of withstanding temperatures higher than approximately 1537.degree. C. In Patent Application EP-A-165,609 it has been considered that the treatment of a hydrogen sulphide-rich gas stream with a stream of air enriched with oxygen to 30-32% could not be carried out in a conventional Claus furnace, that is to say a Claus furnace sized to operate with air not enriched in oxygen. A process has therefore been proposed according to which the effluents originating from the reactions which have taken place in the furnace are cooled. After the sulphur formed during the said reactions has been condensed and extracted, a portion of the cooled effluents is recycled into the reaction zone of the furnace in order to lower the temperature of this reaction zone. Such a process allows a treatment of hydrogen sulphide with highly oxygen-enriched air. However, it has the disadvantage of requiring the use of recycling equipment to be added to a conventional Claus furnace; this process cannot therefore be carried out by means of a conventional Claus furnace.
Another process for treating hydrogen sulphide-rich gas with a stream of oxygen-enriched gas has been described in Patent Application EP-A-237,217. This process consists in partially oxidizing the hydrogen sulphide with pure oxygen or with highly oxygen-enriched air in two stages carried out in two different combustion zones, each separated by a cooling zone. This process permits staging of the heat of reaction between the two zones. However, it has the disadvantage of requiring the addition of a second Claus furnace, and this results in a considerable capital cost. This process, too, cannot therefore be implemented by means of a conventional Claus furnace. From EP-A-315 225 it is also known to inject oxygen, hydrogen sulphide and air into a Claus furnace burner, from the centre outwards. The speed of injection of oxygen is between 50 and 250 m/s, whereas hydrogen sulphide is injected at a speed of between 10 and 30 m/s, which represents a ratio of oxygen/hydrogen sulphide speeds of between 25 and 1.7.